1. Field of the Invention
This invention relates to a single-phase brushless motor which is well adapted for a spindle motor for a floppy disk drive, a motor for audio and video appliances, and a DC brushless axial-flow fan motor.
2. Description of the Prior Art
A brushless motor is used in a variety of appliances, owing to its advantages that it emits little noise and has a long life, because it has neither brush nor commutator, in addition to its characteristics as a DC motor.
In a brushless motor, an electronic circuit is used to switch energization of armature coils of the motor and includes a driving circuit which necessarily includes a number of position detecting elements (normally, magnetic sensors such as Hall effect elements and Hall ICs are used) corresponding to the number of applicable phases of the motor. Accordingly, it is a drawback that as the number of phases increases, the production cost increases accordingly, due to such position detecting elements.
Therefore, inexpensive appliances such as axial-flow fans commonly employ a single-phase brushless motor which includes a driving circuit designed for a single-phase and hence can be produced at a low cost.
Such a single-phase brushless motor has a drawback that it cannot start itself it if stops at a dead point.
Therefore, a conventional single-phase brushless motor is normally provided with means for allowing self-starting of the motor. Referring to FIGS. 1 and 2, single-phase brushless motors including such means are shown and denoted at 1 and 2, respectively. The brushless motors 1, 2 each have an air gap 3 which is partially sloped as at 4 so as to cause a cogging torque to be generated, thereby to assure self-starting of a field magnet 6, 6' having driving magnetic pole zones 5, 5', respectively.
More particularly, FIG. 1 is a developed view of a flattened coreless single-phase brushless motor. Referring to FIG. 1, the field magnet 6 of the brushless motor 1 constitutes a rotor and has six such driving magnetic pole zones 5. The field magnet 6 is mounted for rotation relative to a stator armature 9 which includes a stator yoke 7 having such sloped faces 4 formed on a face thereof and a pair of coreless armature coils 8-1, 8-2 located at same phase positions on the face of the stator yoke 7.
In the single-phase brushless motor 1, when it is deenergized, it is normally stopped at a particular position in which a most projected end 4a of one of the slopes 4 formed on the stator yoke 7 is opposed to a mid-portion of one of N (north) or S (south) magnetic pole zones 5 of the field magnet 6.
Thus, if the armature coils 8-1, 8-2 are located so as to generate a torque in this condition and a magnetic sensor such as a Hall effect element or a Hall IC (integrated circuit) serving as a position detecting element (not shown) is located at any position other than boundary positions between adjacent N and S pole zones of the field magnet, that is, than dead points, the magnetic sensor will provide an output without fail when the brushless motor 1 is to start. Consequently, the armature coils 8-1, 8-2 will be energized in a predetermined direction from a driving circuit in response to such output signal from the magnetic sensor to start rotation of the motor 1 in a predetermined direction.
It is to be noted that reference numeral 10 in FIG. 1 denotes a rotor yoke for closing a magnetic path of the field magnet 6.
Such a flattened coreless single-phase brushless motor 1 has a drawback, in that it is troublesome that the slopes 4 must be formed on the stator yoke 7 and the thickness of the air gap 3 is relatively great due to the slopes 4, and hence a high turning torque cannot be obtained, resulting in low efficiency of the entire motor 1.
Meanwhile, FIG. 2 is an illustration of a 4-magnetic pole, 4-stator pole, 4-coil cored, i.e., with a core, single-phase brushless motor. The single-phase brushless motor 2 includes a stator armature core 13 having 4 radially extending stator poles 11 formed in a circumferentially equidistantly spaced relationship from each other and joined together at base end portions thereof. An armature coil 12 is wound on each of the stator poles 11 of the stator armature core 13.
The field magnet 6' is located in an opposing relationship to the stator armature core 13, with the radial air gap 3 left therebetween and is supported for rotation relative to the stator armature core 13.
In the single-phase brushless motor 2, slopes 16 are formed on the stator poles 11 of the stator armature core 13 to provide slopes to the air gap 3 in order to allow self-starting of the motor 2.
It is to be noted that in the single-phase brushless motor 2 the armature coils 12 are wound in bifilar windings such that two opposing ones of the armature coils 12 for a first phase may be energized with a polarity opposite to the polarity of the other opposing armature coils 12 for a second phase.
It is to be further noted that the single-phase brushless motor 2 is characterized in that stator poles 11 of the stator armature core 13 thereof are each sloped or slanted with respect to the rotor such that the dimension of the air gap gradually increases toward a clockwise direction. The motor 2 is thus relatively simple in construction (but still remains complicated because the slopes 16 must be formed), but it has a drawback that, because the dimension of the air gap must be relatively great, a high torque cannot be obtained and the efficiency of the motor is low.
Further, the single-phase brushless motors of FIGS. 1 and 2 which utilize a cogging torque have another drawback that smooth rotation cannot be attained because a high cogging torque is produced.
FIG. 3 is an illustration of another single-phase brushless motor wherein a relatively high torque can be obtained with a coreless structure by making an air gap uniform without a slope.
The single-phase brushless motor generally denoted at 1' here includes an iron bar 17 arranged in an air gap 3' in order to allow self-starting of the motor 1'.
Due to the presence of the iron bar 17, a field magnet 6 attracts the iron bar 17 and is thus moved to a position from which it can start itself. Accordingly, with a single position detecting element, the single-phase brushless motor 1' can start itself and rotate continuously.
However, if the iron bar 17 is fattened to obtain a higher cogging torque to further assure self-starting of the single-phase brushless motor 1', when it opposes to an S or N pole zone of the field magnet 6, a magnetic flux 18 of the field magnet 6 will pass as illustrated in FIG. 4, but a magnetic flux 18 will pass as illustrated in FIG. 5 when the iron bar 17 is at a position opposing a boundary between adjacent N and S pole zones of the field magnet 6. Thus, sometimes the field magnet 6 may stop, upon deenergization of the motor 1', at such a position as seen in FIG. 5.
Thus, the single-phase brushless motor 1' has a drawback that if the field magnet 6 stops at such a specific position, energization of the armature coils 8-1, 8-2 will not start the motor 1'.
Single phase brushless motors are also known wherein a non-magnetized zone (or a substantially non-magnetized zone) is formed on a driving field magnet. Brushless motors of this type have a drawback that it is difficult and troublesome to form such a non-magnetized zone, which is not suitable for mass production of brushless motors of the type.
A single-phase brushless motor of the type is disclosed, for example, in U.S. Pat. No. 3,299,635 and is shown in FIG. 6.
The single-phase brushless motor generally denoted at 24 in FIG. 6 (a brushless motor of the type may sometimes be called a two phase brushless motor but is properly called a single-phase brushless motor because of its energizing method; this also applies to a brushless motor shown in FIG. 7) is constituted as an inner rotor motor and includes auxiliary stator poles.
A stator armature core 19 has 4 radially extending main stator poles 20 formed in a circumferentially equidistantly spaced relationship, 4 armature coils 21 wound on the main stator poles 20, and 4 smaller auxiliary stator poles 22 formed between the main stator poles 20.
A field magnet 23 is opposed to the stator armature core 19 and has a pair of N pole zones, S pole zones and O pole (non-magnetized) zones successively formed thereon and each having an angular width equal to an electrical angle of 120 degrees.
The single-phase brushless motor 24 shown in FIG. 6 is very useful but has such a drawback as described above due to the presence of the non-magnetized zone.
A single-phase brushless motor in which a substantially non-magnetized zone is formed on a field magnet is shown in FIGS. 7(a) and 7(b).
The single-phase brushless motor generally denoted at 25 is constituted as an outer rotor motor wherein a field magnet 26 as shown in FIG. 7(a) rotors around a stator armature core 27 as shown in FIG. 7(b).
Referring to FIG. 7(a), the field magnet 26 has four driving alternate N and S magnetic pole zones 28 formed by magnetization in a circumferentially equidistantly spaced relationship from each other and eight auxiliary alternate N and S magnetic pole zones 29 also formed by magnetization in a circumferentially equidistantly spaced relationship from each other. Thus, the ratio a:b in width between the driving magnetic pole zones 28 and the auxiliary magnetic pole zones 29 is 2:1, with each of the 4 driving magnetic pole zones 28 overlapping two of the 8 auxiliary magnetic pole zones 29.
Referring now to the FIG. 7(b), the stator armature core 27 has 4 main stator poles 30 and 4 auxiliary stator poles 31 located between the stator poles 30 as in the single-phase brushless motor 24 shown in FIG. 6 (while there is a difference in that the motor 24 of FIG. 6 is constituted as an inner rotor motor and the motor 25 of FIGS. 7(a) and 7(b) is constituted as an outer rotor motor). Armature coils 32 are wound only on the main stator poles 30 and are connected into two phase windings, such that each two opposing ones thereof which are located at symmetrical positions spaced by an angle of 180 degrees relative to the center of the motor 25 are conneted in series, so as to provide a same polarity to the associated main stator poles 30.
A position detecting element such as a Hall effect element is provided at a location not shown for detecting the position of the field magnet 26, in order that the field current of the armature coils 32 is switched by means of a transistor in response to a relative position of the field magnet 26 to the stator armature core 27, to obtain a torque in a predetermined fixed direction.
The single-phase brushless motor 25 having such a construction as described above thus eliminates dead points by the combination of the composite field magnet 26 and the auxiliary stator poles 31. Meanwhile, the torque is generated by the composition of the 4 driving magnetic pole zones 28 and the 8 axuiliary magnetic pole zones 29.
The single-phase brushless motor 25 having such a construction as described above necessitates only one position detecting element an can be reduced in number of circuit components and in size of circuit if it is driven in half-waves. Thus, the single-phase brushless motor of the type is effective in practical use.
However, the single-phase brushless motor 25 also has a drawback similar to that of the single-phase brushless motor 24 described above because the auxiliary magnetic pole zones 29 and substantially non-magnetized zones are formed.
Further, the single-phase brushless motors 24, 25 which include such non-magnetized zones or substantially non-magnetized zones have another drawback, in that their structure is complicated so that they cannot be mass produced and their production cost is high because the field magnets 23, 26 are complicated and the auxiliary stator poles 19, 31 must be formed in order to allow self-starting of the motors 24, 25.
In addition, due to the presence of the auxiliary stator poles 19, 31, it is difficult to wind the armature coils 21, 32 around the main stator poles 20, 30, which makes mass production of motors of this type even more difficult.
It is to be noted that, of the single-phase brushless motors 1, 1', 2, 24, 25 described above, the motors 1, 1' of the coreless type are advantageous in that they have a relatively small number of parts, can be flattened in an axial direction and can be mass produced in a mass at a low cost, but they have a drawback that a high torque cannot be obtained.
On the other hand, the cored single-phase brushless motors 2, 24, 25 have drawbacks that they have a relatively large number of parts and hence their production cost is high and they cannot be flattened in an axial direction, but they are advantageous in that a high torque can be obtained.
Whether a single-phase brushless motor should be of the coreless structure or of the cored structure depends upon an application of the motor and such advantages and drawbacks of motors of the individuals types. Here, where a high torque is required, preferably a single-phase brushless motor of the cored type is used.
However, except single-phase brushless motors of the coreless structure, conventional single-phase brushless motors of the cored type generally have a drawback that the structure is complicated and hence their production cost is high.